The present invention refers to a probe for detecting or monitoring components of interest in a fluid or other material aggregation using membranes. More particularly, the invention relates to a sheet-type membrane probe.
Membrane-based sampling probes can permit analyte collection with good filtering of the base fluid itself or other interferents (i.e. solids) that may affect, damage or interfere with the analyzer or require some further filtering. Membranes used in these probes may be known as permeation or diffusion membranes and may be made from any permeable, semi-permeable or diffusion materials. Membrane-based monitoring processes can easily be made continuous and the probe and the afferent analytical system can be separated from each other. The sample collection can take place through a collector fluid (also called a carrier or collection fluid) flowing between the probe and the analyzer, which collects the analytes, also termed permeates, permeating the membrane at the probe and returns them to the analyzer for analysis.
The use of membrane probes for analytical purposes requires particular parameter adjustments, especially when the analytes are to be extracted from liquids or liquids streams. For example, permeation of the analyte is driven by its partial pressure differential across the membrane wall. In order to keep it at maximal values the inner side of the membrane is swept using a collector fluid, such as for example an inert gas. The collector fluid provides for transport of the collected analytes to the analyzer. Thus, the partial pressure of the analytes cannot be higher than their partial pressure in the sample fluid outside the membrane. In the case of sampling from a gas mixture, the requirement for a partial pressure drop across the membrane translates into lower concentrations of analyte collected than analyte concentration in the sample fluid. Moreover the higher the collector flow rate (in order to achieve a short response time to the analyzer), the lower the concentration collected. These effects result in a membrane attenuation factor due to the collector fluid flow.
In harsh industrial applications such as drilling fluid monitoring, the membrane may be required to withstand very adverse conditions like solid cuttings flowing together with the liquid mud, high pressures, intermittent pressure peaks, high liquid viscosity, etc.
In order for such a membrane-based sampling probe to achieve acceptable performance, it is desired to employ a membrane with a minimal thickness but capable of withstanding accidental mechanical hits and fluid abrasion. The membrane can include an active area in contact on one side with the sample fluid and on the other side with the collector fluid. Oftentimes it is necessary that the probe carrying the membrane be small enough for installation through the wall of a process vessel (pipe tank, reactor, etc). In view of the desirability of a probe having a maximal active area and small insert diameter, longitudinal shaped probes were developed where the diameter of the membrane supporting part is smaller than the diameter of the end fitting. This allowed probe replacement from the outside of the process vessel and facilitated sealing about the probe.
Some previous probes are described in U.S. Pat. Nos. 5,317,932 and 5,442,968 both of Westlake III, et al.; and U.S. Pat. No. 5,469,917 of Wolcott. These probes use capillary tubing shaped membranes laid on grooves in a membrane support body on the probe. The grooves provide a mechanical protection for the membrane tubing. However, the tubing-in-groove geometry forms hidden (dead flow) spaces between the tubing and the groove lateral walls and/or groove roots which spaces limit the active surface area of the membrane. In addition, the undulating surface can create a significant fluid boundary layer through which the analytes must pass in order to reach the membrane, especially, for example, where the sample fluid is a viscous liquid.
The function of tubing-based membrane probes can also be limited by high pressure applications, where the tubing collapses in certain pressures. Pressure collapse withstanding is generally a function of wall thickness, membrane type and tubing inner/outer diameter. However, membrane tubing having higher wall thickness often exhibit poorer permeation characteristics. It is possible to increase a tubing pressure performance by increasing the collector fluid pressure, possibly resulting in unfavorable analyte dilution effects Another challenge in using membrane probe devices for sampling relates to the transport time between the probe and the analyzer. This is especially relevant where the probe and analyzer are required to operate when spaced at more significant distances. In order to obtain a short transport (and implicitly response) time it is oftentimes necessary to increase the collector fluid flow rate. However, in this case an undesirably high head pressure may be required to pass the pneumatic resistance of the tubing or the tubing may tend to fail by blowing out.
The foregoing head pressure problems can be overcome to some degree by providing a plurality of membrane flow paths in parallel on the probe. Such a probe using capillary membranes is described in U.S. Pat. No. 5,553,484 of Bender et al., where multiple capillary tubings are mounted in a parallel flow configuration with each other. However, this probe, and its methods of manufacture and eventual tubing replacement procedures may be quite complex.
Some probes employ sheet-type membranes, rather than membranes of capillary tubing. The use of sheet-type membrane materials may improve the fluid flow characteristics over a probe's outer surface, when compared to capillary-based systems. In addition, there are generally more sheet-type membrane material options than for capillary membranes. Some probes using sheet-type membranes employ a disk shaped membrane geometry. One such embodiment is described in application WO96/07885 of Kristensen wherein the collector fluid flows through a narrow channel formed beneath the disk-shaped membrane. Other probes use a longitudinal membrane shape, termed herein ribbon-type. Such a probe is described in U.S. Pat. No. 6,562,211. While sheet-type membrane probes may address some of the liquid boundary layer problems of capillary type probes, the prior probes using sheet-type membranes continue to experience manufacture and low membrane active permeating surface area difficulties.